A shuttle vector plasmid was constructed which carries a small marker gene and which replicates in human and in bacterial cells. Plasmids treated with ultraviolte light were introduced into repair-proficient and repair-deficient human cells. Plasmids with UV-induced mutation in the marker gene were identified and characterized in a sensitive microbiological assay. The precise nature of the mutations was determined by DNA sequence analysis. The nature and location of changes were different in the repair-proficient and -deficient cells. This is the first molecular determination of carcinogen-induced mutations in human cells.